HIV-1 establishes a persistent infection characterized by ongoing viral replication in the face of vigorous host immune responses, CD4 cell depletion, and ultimately AIDS. For SIV models of AIDS, attenuated SIVs have provided powerful tools to understand pathogenesis, as they are viruses that can be controlled by the host. These models have also provided the best evidence that host immune responses can protect animals from infection and/or disease when challenged with pathogenic isolates. However, for even the best characterized attenuated viruses, the mechanism(s) that underlie their attenuation and the immune correlates of this protection are unknown. Moreover, when protection is achieved, it is typically for homologous rather than heterologous viruses, a scenario far removed from what will be required for an effective vaccine. Our laboratory has shown that mutations in a conserved GYxxX trafficking motif in the SIVmac239 Env cytoplasmic tail produce a profoundly attenuated phenotype that renders this highly pathogenic virus susceptible to host immune control. We have also shown that infected animals are protected from infection with a homologous SIVmac239 challenge, and remarkably, are able to control a heterologous challenge virus (SIVmac E660). We propose to extend our preliminary results in this model to characterize the effects of GYxxX mutations on the virus in vitro and, in rigorous in vivo studies in rhesus macaques, to identify the correlates of reduced pathogenicity and host immune control. Three aims are proposed. Aim #1 will evaluate the effects of TM tail mutations on virion structure, composition, infectivity, and fitness, and will explore the basis for the increased neutralization sensitivity of these viruses that we noted in our preliminary results. Aim #2 will comprehensively evaluate early and late events of infection in mucosal and other lymphoid sites to determine what cells are infected, what cytopathic effects occur, what inflammatory responses are elicited, and whether the attenuation observed in this model affects the ability of GYxxX mutants to be transmitted mucosally. Aim #3 will evaluate cellular and humoral immune responses to GYxxX mutants to determine what differences exist compared to SIVmac239 in breadth, specificity and magnitude. Responses will be assessed before and after challenge with a pathogenic heterologous virus to determine what are the immune correlates of protection. These aims will also take advantage of viruses that have developed apparent compensatory mutations, as they will be powerful tools for future studies to address basic cellular mechanisms that underlie this model of attenuation and host immune control. Collectively, our work shows that rationally designed mutations in Env trafficking signals can be exploited to disrupt the viral/host balance and the outcome of infection. This model should be a highly informative for both pathogenesis and vaccine research.Project Narrative Three major obstacles in the development of an effective HIV vaccine have been 1) the lack of animal models that reproducibly show protection from genetically diverse challenge viruses;2) a poor understanding of what components of the immune response are needed to prevent or control HIV infection;and 3) how a vaccine can be developed that will elicit these immune responses. Our application describes a new model of a live attenuated SIV in which a rationally designed mutation in the viral envelope gene creates a virus that can be completely controlled by the host immune system while conferring the ability to prevent or control infection by pathogenic viruses that are genetically different from the vaccine strain. The proposal describes plans to evaluate the pathological, immunological and virological aspects of this new model with state of the art technologies and addresses issues that are central to HIV vaccine science.